In a coaxial cable based local area network (LAN), communication between nodes occurs over a shared coaxial cable. When the coaxial cable used for the LAN is shared with a community aerial television or cable TV (CATV) service the signals must be separated to avoid interference. The LAN signal can use one band of frequencies and the cable TV service can use a different band. A typical cable TV configuration for a home is shown in FIG. 1. Signal splitters are used to distribute downstream signals from the point of entry (POE) to the various terminals in the home, which can include cable converter boxes, televisions, and cable modems, generally referred to as customer premise equipment (CPE). Each terminal device may have the ability to transmit as well as receive. The upstream signals transmitted by the terminal device flows through the signal splitters back to the POE and into the cable plant and are received by the head end. The signal splitters are functioning as signal combiners for upstream signals. Devices communicating with the head end generally do not communicate with each other.
Signal splitters are commonly used in home and other building type coaxial cable wiring. They have an input port and multiple output ports. The input port can also be considered a common port. The output ports can also be considered tap ports. Splitters are generally passive devices and can be constructed using lumped element circuits with discrete transformers, inductors, capacitors, and resistors. Splitters can also be constructed using strip line or microstrip circuits. A typical two-way splitter splits the power equally between the two output ports if each port is terminated equally. Thus each output would have a power level 3 dB lower than the input. Ideally, a splitter transfers all power from the input port to the output ports. In a practical implementation there is a modest power loss in the splitter due to impedance mismatches, non-zero resistances, dissipative losses in circuit elements, and other non-ideal properties. These losses amount to approximately 0.5 dB, thus a practical two-way splitter provides −3.5 dB power level to each output.
Splitters are generally bi-directional; they can also function as signal combiners, which sum the power from multiple ports into a single output. The ports used as outputs in a splitter configuration become inputs ports for the combining configuration. The common port becomes the output port.
A splitter may have 3 or more tap ports. There is typically an N-way splitter at the point of entry of a building that distributes the incoming CATV signal to each room of the building. The N-way splitter also combines the signals from each room, providing a return path for devices such as cable modems to transmit back to the cable head-end.
Splitters can be designed with power dividing ratios that are not equal. Instead of a 3 dB loss to each port, one port can have, for example 1.15 dB loss, and the other 6 dB. This corresponds to 75%/25% coupling. This type of splitter could be used to balance signal power at all terminal devices when there are multiple levels of signal splitters. A branch that terminates directly to a terminal device would be connected to a higher loss tap port. A branch that contains additional splitters would be connected to a lower loss tap port, which provides extra power to compensate for the loss of additional splitters.
Another characteristic of interest in signal splitters is the isolation between output ports. The isolation is typically between 10 dB and 40 dB. This isolation attenuates signals communicating between tap ports. The signal splitter/combiner is therefore directional, power flows to and from the common port to the tap ports, but power is attenuated between tap ports.
In a conventional cable TV or cable modem use, this isolation is desirable because terminal devices do not communicate with each other, they only communicate through the POE with the cable head-end. In a LAN system, terminal devices must communicate directly with each other, therefore attenuation between tap ports in the signal splitters results in an undesirable signal loss. Presenting a further problem to terminal to terminal communications is the variation in attenuation in different branches of the building wiring. All terminal devices are not connected directly to the main splitter, but may be connected at a secondary splitter, for example in a room. The level of attenuation between different pairs of terminal devices needing to communicate may vary by 10 to 40 dB or more.
Several buildings may be connected to one splitter at the street. Although the isolation of this splitter is high, some coupling between taps occurs, allowing transmission from one building to another. This is a source of interference between LANs in neighboring buildings.
Another approach to overcoming the splitter inter-port isolation is to replace the main splitter at the building POE with a symmetric power splitter/combiner. In a symmetric splitter, power entering any port is divided among the other ports. A symmetric splitter/combiner is not directional. This type of splitter has 3 dB or more additional loss compared to a directional signal splitter. The additional loss is greater depending on the number of tap ports. A power amplifier may be required to boost the signal to compensate for this loss. A bi-directional device, such as a cable modem, requires a reverse path so the amplifier needs to be bi-directional. Another disadvantage to this approach is that installation is required; each coax connected to the existing N-way directional splitter must be disconnected and moved to the new splitter. Another disadvantage of this approach is that power must be available for the amplifier, which is not generally present in the area a typical main splitter is located.
Imperfect terminations at connections create micro reflections within the cable wiring. The dominant main signal and reflected signal combine in the channel. A reflection anywhere in the wiring produces a multipath signal in some or all wiring branches that creates inter-symbol interference (ISI). The multipath signal has a delay and amplitude difference relative to the main signal. In the frequency domain, a reflection produces ripples in the response of the channel, creating amplitude variations across the pass band. In the time domain, ISI is seen as an impairment to the shape of the digital signal pulses. ISI will degrade the bit error rate (BER) performance of the communication channel. To overcome the effects of reflections, an adaptive equalizer is commonly used in the terminal device receiver. This creates a filter response that restores a flat frequency response impaired by the multipath signal.
The signals transmitted by LAN devices can create interference with televisions and set top boxes even though the LAN signals are out of band of these receivers, due to down conversion and signal mixing in the receivers.
Broadband networks are described in U.S. Pat. No. 5,889,765 “Bi-directional communications Network” issued to Gibbs, U.S. Pat. Nos. 5,940,387 and 6,005,861 “Home Multimedia Network Architecture” issued to Humpleman, U.S. Pat. No. 5,870,513 “Bi-directional Cable Network with a Mixing Tap or Suppressing Undesirable Noise in Signals From a Remote End of the Network” issued to Williams, U.S. Pat. No. 5,805,591 “Subscriber Network Interface” issued to Naboulsi, U.S. Pat. No. 6,008,368 “Ethernet Transport Facility Over Digital Subscriber Lines” issued to Rubinstain, U.S. Pat. No. 6,137,793 “Reverse Path Multiplexer for Use in High Speed Data Transmissions” issued to Gorman, and U.S. Pat. No. 6,091,932 “Bidirectional Point to Multipoint Network Using Multicarrier Modulation” issued to Langlais, each of which is incorporated herein by reference.
Gibbs discloses a broadband network overlaid with the cable service frequencies using dynamically allocated TDMA protocols. Humpleman patents disclose a home network using an active network interface unit to couple the home network to the external network. Williams discloses a method of reducing noise accumulated in the frequency bands used by an upstream signal. Naboulsi discloses an active network interface for an asynchronous transfer mode (ATM) network. Rubinstain discloses a method of transporting Ethernet over twisted pair lines. Gorman discloses an active reverse path multiplexer for communication between the cable head-end and subscriber cable modems. Langlais discloses a two-way data transmission system for communicating between an upstream and downstream unit using OFDM.
U.S. Pat. No. 6,091,932 “Bidirectional point to multipoint network using multicarrier modulation” issued to Langlais, incorporated herein by reference, discloses various techniques for implementing OFDM communication. This reference discloses the use of OFDM for communicating between a terminal device and the cable head-end.
U.S. Pat. No. 6,292,651 “Communication system with multicarrier transport distribution network between a head end terminal and remote units” issued to Dapper et al. incorporated herein by reference discloses communication between a cable head end and remote units and discloses techniques using OFDM for such communication. U.S. Pat. No. 5,959,967 “Digital transmission system” issued to Humphrey, incorporated herein by reference, discloses an OFDM transmission system used to communicate over a twisted pair loop. U.S. Pat. No. 5,371,548 “System for transmission of digital data using orthogonal frequency division multiplexing” issued to Williams incorporated herein by reference, discloses an OFDM system for data transmission during the vertical blanking interval of a television signal. U.S. Pat. No. 5,488,632 “Transmission and reception in a hostile interference environment” issued to Mason, incorporated herein by reference, discloses additional techniques for implementing an OFDM modulator and demodulator. U.S. Pat. No. 3,488,445 “Orthogonal Frequency Multiplex Data Transmission System” issued to Chang, incorporated herein by reference, and U.S. Pat. No. 3,511,936 “Multiply Orthogonal System for Transmitting Data Signals Through Frequency Overlapping Channels” issued to Saltzberg, incorporated herein by reference, disclose OFDM data transmission techniques.
A reflected signal and an attenuated signal passing through a splitter port creates a multipath environment. The received power level of the direct signal relative to the reflected signals can vary between equal levels to one signal being substantially greater than the other. The multipath environment impairs the ability to achieve high data rates in a communication network. The signal reflections and tap port isolation of splitters existing in a typical cable TV wiring configuration presents a problem for shared usage of the cable for a LAN system. The prior art references address communicating between a cable head end and in-home units but do not address the impairments present in the home wiring that restricts high bandwidth communication between devices within the home.